Nice of you to fix your neighbors cat! I would have informened him how much it cost and plainly said "I expect you to refund the cost as a responsiable pet owner"

I was thinking that the ones that vanished may also have had owners that started keeping them inside after seeing they had been fixed. (?) They are fair game if turned loose unaltered and it is good you caught them and had that done no matter if they had owners or not. GOOD FOR YOU! Great job...